1. Field of the Invention
This invention relates to a system for use in computer assisted surgery. More specifically, the invention relates to a system for providing visual feedback regarding surgical tool positioning with respect to fluoroscopic images of a body part during an orthopaedic procedure.
The invention also relates to a system for providing the surgeon with improved visual feedback for the positioning of one surgical tool with respect to another surgical tool or implant.
2. Description of the Related Art
Orthopaedic procedures generally involve the fixation of a screw, plate, prosthetic component, or other implant to the bone of a patient. Typically the bone into which the implant is inserted or affixed is only partially exposed to the surgeon's vision. In order to align the implant with respect to the unexposed bone, some sort of imaging modality is required (preoperative x-rays, preoperative CT scans, or intraoperative x-rays using a C-arm fluoroscope). However, these images can be very difficult to correlate to the patient's anatomy in a useful manner. The field of image guided surgery is concerned with the use of computer technology to present these images to the surgeon in a manner that makes them more relevant and useful.
In the case of intertrochanteric hip fractures, the treatment of choice is the insertion of a lag compression screw. The first step in this procedure is the insertion of a guide pin along the intended trajectory of the screw from the lateral femur through the center the femoral head. This has been traditionally performed with repeated images from a C-arm, allowing the surgeon to monitor the alignment and progress of the guide pin insertion. Because x-ray images provide information in only two dimensions, two separate images taken from different positions are required to demonstrate the correct positioning of the guide pin in three dimensions. In practice, this means that the C-arm must be repositioned each time an updated set of images is acquired. Not only does this add to the duration of surgery, but during this time the surgical instrument visible in the existing image may move. Thus it is not guaranteed that two orthogonal images will represent the current pose of the surgical tool. (An object's pose may be defined as its position in space and, to the extent known or calculable, its orientation.) Further, the images that are acquired by the C-arm do not represent linear projections of the anatomy. The image captured by the image intensifier and camera unit of the C-arm is subject to distortions due to both the geometry of the image intensifier and the effect of magnetic fields (including Earth's magnetic field) on its internal electron beam. These cause a "warping" of the image and lead to straight objects appearing curved in the x-ray images. Further, the degree of distortion varies with respect to several factors including C-arm orientation, image intensifier shielding and size and proximity of ferrous objects. Other factors, such as rigidity of the source/receiver connecting structure and operating temperature, as well as magnetic fields, induce a translational offset to the image.
This inability to obtain accurate and linear images, simultaneously in two views, may lead the surgeon to insert the guide pin along a path other than the intended one. These misplaced attempts can add significantly to the duration of the surgery and the amount of radiation exposure to OR personnel as well as compromise of the bone stock. Further, the risks of a prolonged procedure and the difficulty of inserting a guide pin near the hole from a previous failed attempt may lead the surgeon to accept a pin position that is suboptimal. A serious complication, the "cutting out" of the screw through the femoral head into the hip joint, has been linked in numerous studies to poor placement of the screw.
Several image guided systems have been proposed to deal with the problems of this and similar surgeries. U.S. Pat No. 5,517,990, Kalfas, et. al., May 21, 1996, describes an image guided surgical system that is similar in concept to the majority of systems currently in use. This system uses sonic tracking of a probe to navigate CT data of the patient's head or spine. However, CT scans are not indicated for most orthopaedic trauma procedures and would add significantly to the cost of treatment if obtained. Further, CT scans must be registered to the bony anatomy (i.e., a mathematical relationship must be found between the coordinate frames of the CT scan and of the bone). This requires an intraoperative step in which a probe is used to sample the positions of landmarks on the bone as these same points are selected in the imaging data. (Alternatively, small radiopaque markers may be used as landmarks.) Such systems and their complicated user interfaces are often found by surgeons to be time consuming and difficult to use.
Another image guided system has been described in U.S. Pat. No. 5,772,594, Barrick, Jun. 30, 1998. This system displays the pose of a surgical tool over intraoperative fluoroscopic images during hip screw placement. This system, however, requires that the bone be registered to the images by finding small, different shaped, radiopaque markers. This introduces extra steps to the process and may negate the potential time savings. Also, no method is described for the correction of the nonlinearities present in the C-arm images.
Another solution for the difficulties in hip screw placement is proposed by Phillips, et. al. They describe a fluoroscopic system wherein image processing techniques are used to identify a path for the guide pin. The surgeon then implements this by aligning the tool, connected to a passive manipulator, until crosshairs on the display align. The drawback of this system is that it uses the surgeon as an assistant to implement its plan instead of providing improved information to the surgeon with which to plan and execute the procedure.
Another application for the system proposed by Phillips, et. al. is the insertion of a screw through a transverse hole in the distal end of an intramedullary (IM) rod that has been inserted down the central canal of a fractured femur. In order to insert this locking screw, a hole is drilled in the bone exactly at the location of the transverse hole with the same orientation. Currently the surgeon aligns the C-arm with the transverse holes so that they appear as "perfect circles" in the images. The surgeon then uses repeated images to align the tip of the drill with the center of the hole while using the C-arm source and receiver as external reference points to correctly orient the drill. This procedure involves numerous x-ray images and often requires several attempts before the screw hole is acceptably placed.
The biggest drawback with using a C-arm to position a drill for IM rod screw insertion is the difficulty encountered in achieving the accurate orientation of the drill in the axial plane. External jigs, attached to the exposed proximal end of the IM rod, have been proposed to assist in the placement of the distal screw holes, but these are unable to account for flex of the IM rod in the bone and therefore are not very useful. The system proposed by Phillips, et. al. extracts features from fluoroscopic images of the inserted IM rod and uses image processing techniques to calculate the trajectory required to pass a drill through the hole. The surgeon then moves a drill guide attached to a passive manipulator until the proper position is achieved and then drills the hole. Again, the drawback of this system is that it uses the surgeon as an assistant in implementing its plan instead of providing improved information to the surgeon with which to plan and execute the procedure.
A similar difficulty encountered by surgeons is the accurate placement of a hole or guide pin through an irregularly shaped or partially obscured bone when fluoroscopic guidance is not used. For example, when drilling holes through the patella for tendon or fracture repairs or the calcaneous for fracture fixation, it may be difficult to correctly align the drill with the intended exit point. The system described in U.S. Pat. No. 5,305,203, Raab, Apr. 19, 1994, includes a means for implementing a previously specified drill trajectory as part of a menu driven surgical system. A drawback of this system is the sequential nature of the indication of the entry point, the indication of the exit point and the implementation of the trajectory by a single passive manipulator arm.
Many of these systems often suffer from a lack of readiness for the operating room. As academic or conceptual systems they do not always address practical considerations. Many systems introduce extra equipment and operative steps to the surgical procedures that prolong the surgery and require significant training. Further, most of the systems do not address the issues of sterility, error checking and safety, and unwanted motion of the body part to be operated upon.
Most systems require input from the surgeon in order to specify data or alter program flow. Many systems rely on a non-sterile assistant to enter data at a keyboard or with a mouse, but this is inefficient and risks miscommunication. A sterilized or draped input device introduced into the surgical field may be difficult to use and distracting for the surgeon. Visarius describes an input scheme in which the surgeon points to fields on a tracked, sterile "virtual keyboard" with the surgical tool. The input scheme described in U.S. Pat. No. 5,230,623, Guthrie, Jul. 27, 1993 uses the surgical tool pointing to an area in space to move a mouse cursor on the screen via an "imaginary mathematical correspondence". Both, however, require the splitting of the surgeon's attention between the display screen in one location and the surgical tool in another as well as the removal of the tool from the surgical site for use elsewhere as an input device.
In order that any motion of the body part which is being operated upon not affect the accurate superposition of the tool on the image data, many systems use a dynamic reference frame. U.S. Pat. No. 5,383,454, Bucholz, Jan. 24, 1995, describes the measurement of all surgical tool poses relative to a ring or to markers attached to the patient's head. This allows the registration between the three dimensional image data and the patient's skull, as well as the accurate positioning of the tool relative to the head, to be maintained despite motion of the head. However, some surgeries, especially orthopaedic trauma procedures, involve multiple body parts (e.g., bone fragments, soft tissue). While not freely mobile, these untracked body parts may experience significant motion if associated structures are moved excessively.